Erasure method for memory-type EL display devices

ABSTRACT

An improved method of completely erasing images stored by memory-type electroluminescent display panels. The method comprises interrupting the a.c. sustaining bias signal normally applied to the panel and impressing an erase pulse that is opposite in polarity to the last-preceding sustaining pulse or cycle. The amplitude and duration of the erase pulse are chosen to effect relaxation of the EL layer&#39;s internal field. The sustaining bias signal is then restored to place the panel in an erased, ready-to-write condition.

BACKGROUND AND OBJECTS OF THE INVENTION

The present invention relates generally to thin-film electroluminescent(EL) display devices of the type having an inherent "memory", or voltagevs. brightness hysteresis. More particularly, the invention relates toan improved method of erasing such devices.

Reports of long service life and an inherent memory capability havefocused increased attention on a.c.-driven ZnS:Mn thin film EL devicesin recent years. The hysteretic behavior of the brightness vs. appliedvoltage relationship of such devices (which is responsible for thememory effect) was first described by Y. Yamauchi et al. in the 1974International Electron Devices Meeting Digest, pp. 348-351. Since thattime, memory-type ZnS:Mn thin film EL devices have been used in avariety of information display applications. For example, C. Suzuki etal. have published descriptions of X-Y matrix EL panels with inherentmemory for displaying alphanumeric characters (1976 S.I.D. InternationalSymposium Digest pp. 50, 51) and television pictures (InformationDisplay, Spring, 1977, pp. 14-19). U.S. Pat. No. 4,149,108 to I. Changdiscloses a multistable cathode-ray tube that uses an a.c. thin-film ELpanel as the storage display target. The electroluminescent target ofthe Chang CRT is addressed directly by the tube's electron beam. U.S.application Ser. No. 922,950, filed July 10, 1978 in my name andassigned to the assignee of the present invention, describes an ELstorage CRT with a target comprising a double insulated ZnS:Mn thin-filmpanel having a layer of a UV-emitting phosphor on the input side. Ratherthan being exposed to direct bombardment by the electron beam the ELpanel is addressed by ultraviolet light emitted from the intermediatephosphor layer.

With all such devices there is a need to erase stored information. Theconventional method of erasing an a.c. thin-film EL display panel is toreduce the amplitude of the applied a.c. signal, which is normallymaintained at an image-sustaining level Vs, to a value below theextinction voltage Ve of the device. At or below Ve, the a.c. fieldacross the thin-film EL layer is insufficient to maintain luminescenceand the entire panel becomes dark, (unwritten). The erase cycle iscompleted by raising the applied signal potential to Vs, which returnsthe panel to an erased, ready-to-write condition.

Although it is easy enough to do, the above-described procedure has asignificant drawback--a faint, residual image of the previouslydisplayed information often remains when the applied voltage is returnedto Vs. The residual image problem can be eliminated by adding apreliminary step to the erasing process: increasing the appliedpotential to a value Vw high enough to place the entire panel in abright, fully-written condition. Then, when the a.c. voltage is droppedto Ve and returned to Vs, the panel will be fully erased and free ofresidual images. Unfortunately, however, the added step produces amomentary bright "flash" that is unpleasant and greatly increases visualfatigue. The effect is particularly objectionable with large sizedisplay panels, or displays used in low ambient light conditions.

Thus, there is a demonstrated need for an improved method of erasingmemory-type electroluminescent display devices--one that is free fromthe drawbacks of the methods described above. A principal object of thepresent invention is to meet this need in a practical and satisfactorymanner.

A more specific object of the invention is to provide a method forerasing memory-type EL panels that results in complete erasure of allpreviously-displayed information without increasing visual fatigue.

Another object of the invention is to provide an erasure method that issimpler and less costly to implement than prior art methods.

Still another object of the invention is to provide an improved methodof total erasure that does not require any change in the amplitude ofthe a.c. voltage applied to the panel.

A further object of the invention is to provide an improved erasuremethod that is applicable in all memory-type thin-film EL panelapplications.

SUMMARY OF THE INVENTION

According to a preferred embodiment of the present invention, which isdescribed in greater detail below, total erasure of a memory-type ZnS:Mnthin-film EL panel is achieved by applying an erase pulse opposite inpolarity to the last-preceding pulse or cycle of the sustaining biassignal, and of sufficient amplitude and duration to relax the internalfield across the EL layer. The sustaining bias is then reapplied toplace the panel in an erased and ready-to-write condition. The polarityof the first sustaining bias pulse or cycle following the erase pulse isunimportant and may be of the same or the opposite polarity. For thesake simplicity and convenience, the erase pulse may be of the sameamplitude as the pulses of the sustaining bias signal.

Other features, advantages and objects of my invention will be apparentas the following detailed description is read in conjunction with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a fragmentary, cross-sectional view of a memory-typeelectroluminscent display panel,

FIG. 2 shows a plot of display brightness versus applied voltage for theFIG. 1 EL panel;

FIG. 3 illustrates the waveform of voltage pulses applied to the FIG. 1panel during a complete erase cycle; and

FIG. 4 depicts the internal electric field produced in the active layerof the EL panel by application of the FIG. 3 waveform.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 shows a thin-film electroluminescentdisplay panel 10 of known construction. Panel 10 includes a transparentfirst electrode 14 of indium oxide (In₂ O₃) or indium-tin oxide (InSnOx)deposited on a planar glass substrate 12. Overlaying electrode 14 is afirst insulating layer 16 of a high dielectric strength material such asyttrium oxide (Y₂ O₃), silicon nitride (Si₃ N₄) or aluminum oxide (Al₂O₃). An active thin-film layer 18 of maganese-activated zinc sulfide(ZnS:Mn) deposited on the first insulating layer is covered by a secondinsulating layer 20 of Y₂ O₃ or the like. A second electrode 22,suitably of aluminum, overlays insulating layer 20 and completes thebasic structure. The fabrication of such panels is conventional, havingbeen described in the above-mentioned Yamauchi et al. paper and numeroussubsequent publications, and forms no part of the present invention. Byway of example only, a typical thin-film EL panel of thedouble-insulated described type may be constituted as follows:

    ______________________________________                                        Element         Materials  Thickness, A                                       ______________________________________                                        1st. Electrode 14                                                                             SnO.sub.2  300-500                                            1st. Insul. Layer 16                                                                          Y.sub.2 O.sub.3                                                                          2000-3000                                          Active Layer 18 Zns:Mn     3500-7000                                          2nd Insul. Layer 20                                                                           Y.sub.2 O.sub.3                                                                          2000-3000                                          2nd Electrode 22                                                                              Al          400-1000                                          ______________________________________                                    

Such a device emits yellow light when activated, and exhibits ahysteretic luminance vs. voltage behavior that is illustrated (insomewhat idealized form) in FIG. 2. As the peak amplitude of applieda.c. voltage pulses (as from a source 24) is increased, there is a slowincrease in emitted light until the voltage exceeds a certain thresholdvalue, indicated at point "a" on the curve. Above this "upward"threshold value, luminance increases very rapidly as trapped electronsare liberated from the active EL layer. Liberation of the trappedelectrons is essentially complete at point "b" on the curve, and furthervoltage increases (to point "c" and beyond provide only small gains indisplay brightness. Upon reducing the amplitude of the applied voltagepulses, luminance decreases at a slow, or relatively constant rate untila "downward" threshold value is reached at point "d", where trappingrapidly reduces the electrons available for excitation of the Mn atoms.Below point "d", further small reductions in the applied voltage rapidlydecrease the device's luminance, until at point "e" the panel is in a"dark" , or erased, state.

For operation as a storage display device, a train 30 of sustained,alternating polarity voltage pulses Ps (FIG. 3) of an amplitude Vs isapplied to panel 10 by source 24. The value of Vs, which may be referredto as the bias voltage, is set in the vicinity of thepreviously-mentioned upward threshold value. Before information iswritten on the panel, its luminance is sustained by the pulse train at arelatively low level, Ls, as shown in FIG. 2. Areas of the EL layer thatare subsequently switched "on" by a writing electron beam, ultravioletlight or a transient voltage pulse settle at a high luminance level, Lw,and are sustained there by the pulses Ps.

According to the present invention, panel 10 is totally erased withoutleaving residual images by interrupting the train of sustaining pulsesand applying an erase pulse opposite in polarity to the last-precedingpulse Ps. The erase pulse amplitude and duration are selected to producerelaxation of the internal field across EL layer 18. Following the erasepulses, the train of sustaining pulses is reapplied to place the panelin a ready-to-write condition.

A representative erase cycle for a memory-type thin-film EL displaypanel is shown in FIG. 3. It will be assumed that the panel initially isin a fully written condition, and thus is being sustained at luminancelevel Lw by the alternating polarity pulses Ps forming pulse train 30.Following a positive sustaining pulse 32, the train is interrupted by anegative erase pulse 34. The amplitude and the duration of the erasepulse are emperically selected to relax the internal field across thethin-film EL layer before the sustaining pulse train is reapplied. Tosimplify implementation of the method, the amplitude Vr of the erasepulse may be the same as that of the sustaining pulses, but should notexceed Vs. Upon the conclusion of pulse 34, a train of sustaining pulsesPs is reapplied to the display panel. The train may begin with a pulseof either polarity without affecting erasure of the panel.

A plot of the internal electric field in the ZnS:Mn active layer duringthe FIG. 3 erase cycle is shown in FIG. 4. As will be observed, eachsustaining pulse Ps steps the layer's internal field by an amount Es,which adds to the polarization field Ep during the next pulse to enhanceluminance. Since the thin-film panel is a capacitive device, erase pulse34 steps the internal field by an amount Er. This increases the field,causing the EL layer to emit light and discharge. The higher the field,the greater the emission and the faster the discharge, which reduces theEL layer's internal field. When the erase pulse terminates and the nextsustain pulse Ps is applied, the total field is insufficient to sustainelectroluminescence at the Lw level. Thus, the active layers freeelectrons are trapped out and the panel returns to the lower luminancelevel Ls. The FIG. 4 plot suggests, as one would expect, that there is aminimum value for Vr, below which erasure does not occur. This valuewill vary, depending on the construction of the EL panel, and is bestdetermined by trial and error.

Visually, erasure of an EL panel by the method of the invention is seenas a rapid, complete disappearance of written images without anyannoying flash. The described technique is applicable to any type ofhysteretic thin-film EL device, including those incorporating a layer ofultraviolet-emitting phosphor overlying the second conductive layer forconversion of incident electrons to UV light. Such a device, used as aCRT storage target, is described in my above-mentioned application, Ser.No. 922,950. One feature of the invention applicable only to aUV-addressed hysteretic EL panel is that writing can take place duringthe erase pulse. Portions of the panel thus written will remain stored,while all previously written information will be erased.

I claim as my invention:
 1. A method of operating an electroluminescentdevice exhibiting voltage/luminance hysteresis, said device comprisingan active layer of electroluminescent material, first and secondelectrodes of conductive material, each electrode being disposedopposite a different face of the active layer, and first and seconddielectric layers separating and insulating said electrodes from theactive layer, said method comprising the steps of:(a) applying asustaining voltage signal to the active layer through said electrodes,said signal including alternating polarity pulses having an amplitudesufficient to bias the device into its region of voltage/luminancehysteresis, (b) discontinuing application of the sustaining signal, (c)applying an erase pulse that reduces the active layer's internalelectric field to a level such that reapplication of the sustainingvoltage signal will cause luminescent regions of the device to beextinguished, said pulse having (i) an amplitude not exceeding that ofthe sustaining signal, (ii) a duration greater than that of theindividual pulses in said sustaining signal, and (iii) a polarityopposite that of the last preceding pulse of said signal, and (d)reapplying the sustaining signal.
 2. The method of claim 1, wherein saidactive layer consists essentially of manganese-activated zinc sulfide.3. The method of claim 1, wherein said sustaining voltage signal is inthe form of a train composed of spaced, alternating polarity pulses. 4.The method of claim 3, wherein said pulses have a substantiallyrectangular waveform.
 5. The method of claim 1, wherein said erase pulsehas an amplitude substantially equal to that of said sustaining voltagesignal.
 6. A method of erasing an electroluminescent display device ofthe type exhibiting voltage/luminance hysteresis, said device comprisingan active layer of electroluminescent material, first and secondelectrodes of conductive material, each one disposed opposite adifferent face of the active layer, and first and second dielectriclayers separating and insulating said electrodes from the active layer,said device including at least one region of luminescence sustained bythe application of alternating polarity voltage pulses to the activelayer through said electrodes, said method comprising the steps of:(a)interrupting the application of said alternating polarity sustainingvoltage pulses to said active layer, (b) applying an erase pulse that is(i) opposite in polarity to the last preceding sustaining pulse, (ii)longer in duration than individual sustaining pulses, and (iii) equal orlower in amplitude than the sustaining pulses, the amplitude andduration of the erase pulse being selected to provide a reduction in theactive layer's internal electric field sufficient to produce extinctionof luminescence in said region upon reapplication of said sustainingvoltage pulses, and (c) reapplying said sustaining voltage pulses. 7.The method of claim 6, wherein said erase pulse has an amplitudesubstantially equal to that of said sustaining pulses.
 8. The method ofclaim 6, wherein said sustaining voltage pulses have a substantiallyrectangular waveform.